1. Field of the Invention
This invention relates generally to wheel trim and more particularly to a lug nut engaging retention system which resists degradation at elevated wheel temperatures.
2. Antecedents of the Invention
Plastic wheel trim have been employed for several years. Among the attributes of plastic wheel trim are resistance to water, road salt and other environmental factors, which effected corrosion in metal wheel trim.
A primary advantage of plastic wheel trim was that it was light in weight and thus did not significantly contribute to total vehicle weight. As such, the employment of plastic wheel trim was one of several factors contributing to increased vehicle mileage efficiency.
Further, plastic wheel trim was moldable into a variety of attractive configurations and was susceptible to any of a variety of outer finishes including metal plating and painting. A significant problem with respect to plastic wheel trim was obtaining satisfactory retention to the wheel. A variety of systems have been proposed for retaining plastic wheel trim with varying degrees of success.
Wheel trim have been secured to the outer wheel weight rim of a wheel, however, metal barbs were required which had a tendency to scratch the wheel and induce wheel corrosion.
It has been found that plastic wheel trim retention systems which engage a wheel lug nut have proven most effective. In U.S. Pat. No. 4,998,780, issued to Martin E. Russell, et al., disclosed a wheel trim retention comprised a series of plastic caps which were captured in apertures extending through the wheel trim. The caps included internal threads which engaged specially configured wheel lug nuts having mating external threads.
While the retention system disclosed in U.S. Pat. No. 4,998,780 proved to be effective in securely retaining the wheel trim, vehicle owners found that the removal and mounting of the wheel trim to a vehicle wheel was not a simple task, since a lug nut wrench was required to rotate the caps. Further, because of elevated operating temperatures generated by vehicle braking systems, plastics having elevated softening temperatures were required. With elevated temperatures, the face of the wheel cover softened, warped and lost its preload, resulting in disengagement of the wheel cover periphery from the wheel rim and concomitant rattling, squeaking and other noises. One approach toward resolving this problem was to increase the thickness of the wheel cover face, increasing both weight and cost.
Push on retention systems have been preferred because of their simplicity in mounting and dismounting the wheel trim, however, the engagement between a plastic retention and a lug nut suffered significant problems because wheel heat, generated by the vehicle braking system, resulted in elevated temperatures not only of the wheel disc, rotor or brake drum, but additionally, the lugs and the lug nuts. The plastic retainers which were in contact with the lug nuts were thus subject to elevated temperatures, which resulted in degraded retentivity and the loss of the wheel trim on a highway, freeway or other roadway and concomitant hazards to motorists attempting to avoid the road debris.
While attempts have been made to employ metal socket clips to grip lug nuts, as exemplified in U.S. Pat. No. 4,133,583 and U.S. Pat. No. 5,297,854, such approach resulted in increased manufacturing costs. Difficulty was often encountered in mounting the wheel trim to the wheel because the metal clips did not provide sufficient guidance for centering the wheel trim on the wheel, with the wheel trim having a tendency to float. Unless costly alloys were employed, the clips were subject to corrosion from water and road salt which was accelerated by the elevated temperatures of the lug nuts.